Investigation into alternative wheat aphid control strategies for emerging farmers
Richter, Johannes Mattheus
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In the Qwa-Qwa and Thaba N’chu regions of the Free State Province, South Africa, resource limited farmers that produce wheat are mainly situated in temporary crop environments. They are drastically affected by crop losses that occur during years of serious Russian wheat aphid (Diuraphis noxia) (Kurdjumov) and oat aphid (Rhopalosiphum padi) (Linnaeus) infestations. Therefore the main objective of this study was to identify simple alternate control methods to be used by small-scale farmers for the control of these aphids. The focus was on minimizing the numbers of the immigrating individuals. That must happen before they arrive in the crop habitat and decrease the possibility of the pest population reaching damaging levels when the crop is still in its susceptible phase for insect damage. Plant derived semiochemicals, which could modify insect behaviour, were considered as an option to be used since this could be extracted from plants, and were demonstrated to be successful in other countries. These semiochemicals are also known to attract natural enemies of these insects. It was therefore decided to test two types of extracts (an aqueous and a light mineral oil) which could be easily prepared from four plant species, namely Wild wormwood Artemisia afra (Jacq. ex Willd.), Big thorn apple Datura stramonium (Linnaeus), Khaki bush Tagetes minuta (Linnaeus) and Wild garlic Tulbachia violacea (Harv.). The plants were chosen due to their availability in the wheat production regions and their possible insect repelling properties known from other species in the same genera. The behavioural response of alate aphids D. noxia and R. padi and two parasitoids, Aphelinus hordei (Kurdjumov) and Diaeretiella rapae (McIntosh) to these extracts was tested in olfactometer trials in the laboratory. The aphid D. noxia showed the highest repellence to the aqueous extract of A. afra and the oil extract of T. violacea. Aphid R. padi was also best repelled by the aqueous extract of A. afra and the oil extract of D. stramonium. The parasitoid A. hordei was strongly attracted to the aqueous extracts of A. afra and T. minuta. Diaeretiella rapae on the other hand, was also highly attracted to the aqueous extract of T. minuta, but T. violacea oil extract had a very strong effect on the parasitoid and would be recommended to farmers. Artemisia afra and T. violacea are perennials and available as green material for extraction purposes in the winter when wheat is planted. The other two plants are annuals and not available in winter. The A. afra aqueous extract will repel both aphid species when sprayed early in the wheat growing season when wheat is still small and aphids are flying into the wheat. This extract will also attract A. hordei and this could enhance the biological control of D. noxia. The T. violacea oil extract could also be used to repel D. noxia. It could also be used to attract the parasitoid D. rapae later in the season and enhance the biological control of both aphid species. Thus there are potential alternate simple aphid control methods available for small-scale farmers. These methods should be refined and farmers trained to use them effectively.